libxmlsec: rewrite README now that we no longer patch the library

Change-Id: I8900e399df970056c8ac781c05e9eff6a43c31c7
Reviewed-on: https://gerrit.libreoffice.org/64195
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

external/xmlsec/README

-XML signing, etc. From [http://www.aleksey.com/xmlsec/]. Heavily patched.
-
-The XML Security library has been modified, so that there is NO verification of
-the certificate during sign or verification operation. On Windows this was done
-in the function xmlSecMSCryptoX509StoreVerify (file src/mscrypto/x509vfy.c) and
-on UNIX in xmlSecNssX509StoreVerify (file src/nss/x509vfy.c).
-
-The implementation creates certificates from all of the X509Data children, such
-as X509IssuerSerial and X509Certificate and stores them in a certificate store
-(see xmlsec/src/mscrypto/x509.c:xmlSecMSCryptoX509DataNodeRead). It must then
-find the certificate containing the public key which is used for validation
-within that store. This is done in xmlSecMSCryptoX509StoreVerify. This function
-however only takes those certificates into account which can be validated. This
-was changed by the patch xmlsec1-noverify.patch, which prevents this certificate
-validation.
-
-xmlSecMSCryptoX509StoreVerify iterates over all certificates contained or
-referenced in the X509Data elements and selects one which is no issuer of any of
-the other certificates. This certificate is not necessarily the one which was
-used for signing but it must contain the proper validation key, which is
-sufficient to validate the signature. See 
-http://www.w3.org/TR/xmldsig-core/#sec-X509Data 
-for details.
-
-There is a flag XMLSEC_KEYINFO_FLAGS_X509DATA_DONT_VERIFY_CERTS that can be set
-in a xmlSecKeyInfoCtx (see function xmlSecNssKeyDataX509XmlRead, in file
-src/nss/x509.c), which indicates that one can turn off the validation. However,
-setting it will cause that the validation key is not found. If the flag is set,
-then the key is not extracted from the certificate store which contains all the
-certificates of the X509Data elements. In other words, the certificates which
-are delivered within the XML signature are not used when looking for suitable
-validation key.
-
+XML signing, etc. From [http://www.aleksey.com/xmlsec/].
 
+The certificate vertification functionality of libxmlsec is not used, both the
+mscng and nss backends specify the
+XMLSEC_KEYINFO_FLAGS_X509DATA_DONT_VERIFY_CERTS flag during verification.